A system working with attracting magnetic forces is in itself unstable. In an assumed state 1, for example, with a given gap width the magnetic holding force supplied by the magnetic device is exactly so great that it is in equilibrium with external forces acting on the vehicle. If the external forces acting on the vehicle should now vary, in an assumed state 2, the magnetic holding force supplied by the magnetic device is then either too great or too small, so that the gap might disappear or an unhindered increase of its size might occur. For this reason an additional-force device acting between the roadway and the vehicle is provided which supplies the necessary force difference between the magnetic holding force of the magnetic device and the total holding force required in consequence of the external load. In this way the gap between the first part and the second part of the magnetic device is kept at a determined magnitude or within a determined range of magnitudes.
Preferred additional-force devices are wheels and/or electromagnets, particularly electromagnets whose magnetic force is variable. However, it is also possible to provide repelling permanent magnets, air cushions, sliding devices, electrodynamic repulsion, and the like. The forces supplied by the additional-force device may be so directed that they assist the attracting magnetic forces, counteract them, or act in both these directions. When wheels are used for example as additional-force devices the action can be applied in both directions by two rolling surfaces on opposite sides of the wheels or by means of wheels which are supported in one direction on a rail, in conjunction with wheels which are supported in the other direction on the same rail or on another rail.
For the purpose of the invention, by the expression "holding a vehicle in relation to the roadway" is understood: substantially vertical holding relative to forces acting substantially vertically, for example the weight of the vehicle including the pay load, inertia forces, disturbing forces; and/or substantially horizontal holding in relation to forces acting substantially horizontally, for example inertia forces on bends, wind forces, other disturbing forces; and/or holding against any other forces which could bring the vehicle out of the intended relative position with respect to the roadway. The holding of the vehicle in relation to the roadway should be effected in such a manner that the gap width between the first part and the second part of the magnetic device remains within a determined range.
If the external forces acting on the vehicle can vary very considerably in magnitude, for example when changing over from an unladen vehicle to a fully laden vehicle, it is advantageous for the gap width between the first part and the second part of the magnetic device to be adapted to the external forces at the moment in question or to the total holding power required at the moment in question, in order that only the lowest possible additional force would have to be supplied by the additional-force device. If external forces are high it will thus be attempted to reduce the width of the gap, while with lower external forces an enlargement of the gap width is advantageous.